Effect of inhibition of angiotensin converting enzyme and/or neutral endopeptidase on vascular and neural complications in high fat fed/low dose streptozotocin-diabetic rats

Abstract

Treating high fat fed/low dose streptozotocin-diabetic rats; model of type 2 diabetes, with ilepatril (vasopeptidase inhibitor, blocks neutral endopeptidase (NEP) and angiotensin converting enzyme (ACE)) improved vascular and neural functions. Next, studies were performed to determine the individual effect of inhibition of NEP and ACE on diabetes-induced vascular and neural dysfunctions. High fat fed rats (8 weeks) were treated with 30 mg/kg streptozotocin (i.p.) and after 4 additional weeks, were treated for 12 weeks with ilepatril, enalapril (ACE inhibitor) or candoxatril (NEP inhibitor) followed by analysis of vascular and neural functions. Glucose clearance was impaired in diabetic rats and was not improved with treatment although treatment with ilepatril or candoxatril partially improved insulin stimulated glucose uptake by isolated soleus muscle. Diabetes caused slowing of motor and sensory nerve conduction, thermal hypoalgesia, reduction in intraepidermal nerve fiber (IENF) profiles and impairment in vascular relaxation to acetylcholine and calcitonin gene-related peptide (CGRP) in epineurial arterioles of the sciatic nerve. Inhibition of NEP improved nerve conduction velocity and inhibition of NEP or ACE improved thermal sensitivity and protected IENF density. Ilepatril and candoxatril treatments of diabetic rats were efficacious in improving vascular responsiveness to acetylcholine in epineurial arterioles; whereas all three treatments improved vascular response to CGRP. These studies suggest that inhibition of NEP and ACE activity is an effective approach for treatment of type 2 diabetes neural and vascular complications.

Fig. 1. Effect of treating high fat/streptozotocin diabetic rats with enalapril, candoxatril ilepatril on glucose tolerance

Glucose tolerance was determined as described in the Methods section. Data are presented as the mean ± S.E.M. in mg/dl. The area under the curve was significantly different for high fat/streptozotocin diabetic rats (p < 0.01), high fat/streptozotocin diabetic rats treated with enalapril (p < 0.01), candoxatril (p < 0.05) or ilepatril (p < 0.05) vs. control. The number of rats in each group was the same as shown in Table 1.

Fig. 2. Effect of treating high fat/streptozotocin diabetic rats with enalapril, candoxatril ilepatril on motor and sensory nerve conduction velocity

Motor and sensory nerve conduction velocity was examined as described in the Methods section. Data are presented as the mean ± S.E.M. in m/sec. The number of rats in each group was the same as shown in Table 1. * p < 0.05 compared to control rats; + p < 0.05 compared to diabetic rats; ‡ p < 0.05 compared to diabetic + enalapril rats.

Fig. 3. Effect of treating high fat/streptozotocin diabetic rats with enalapril, candoxatril ilepatril on thermal nociception and intraepidermal nerve fiber density

Thermal nociception and intraepidermal nerve fiber density was examined as described in the Methods section. Data are presented as the mean ± S.E.M. for thermal nociception in sec and intraepidermal nerve fiber profiles per mm. The number of rats in each group was the same as shown in Table 1. * P < 0.05 compared to control rats; + P < 0.05 compared to diabetic rats.

Fig. 4. Effect of treating high fat/streptozotocin diabetic rats with enalapril, candoxatril ilepatril on vascular relaxation by acetylcholine in epineurial arterioles

Pressurized arterioles (40 mm Hg and ranging from 60–100 μm luminal diameter) were constricted with U46619 (30–50%) and incremental doses of acetylcholine were added to the bathing solution while recording steady state vessel diameter. Data are presented as the mean of % relaxation ± S.E.M. For these studies two vessels were collected from each rat, studied and the data combined. The number of rats in each group was the same as shown in Table 1. * P < 0.05, compared to control; + P < 0.05, compared to diabetic rats.

Fig. 5. Effect of treating high fat/streptozotocin diabetic rats with enalapril, candoxatril ilepatril on vascular relaxation by calcitonin gene-related peptide in epineurial arterioles

Arterioles were treated as described in Fig. 4. Incremental doses of calcitonin gene-related peptide (CGRP) were added to the bathing solution while recording steady state vessel diameter. Data are presented as the mean of % relaxation ± S.E.M. The number of rats in each group was the same as shown in Table 1. * P < 0.05, compared to control rats; + P < 0.05, compared to diabetic rats.